1. The Field of the Invention
The present invention relates to the manufacture of molded articles. More particularly, the present invention relates to systems for precisely metering and delivering a desired amount of a moldable composition into a mold, such as aqueous, starch-based compositions.
2. The Relevant Technology
There has been a marked trend in recent years toward finding biodegradable and environmentally friendly alternatives to containers made from polystyrene foam and other plastic materials. The search for environmentally friendly substitutes has been driven by the recognition that polystyrene and other plastics tend to persist in the environment for years, decades, and even centuries before finally breaking down. Some companies, like McDonald""s Corporation, have abandoned fast food boxes, cups and other containers made from polystyrene foam and plastic in favor of alternative materials.
More bio-friendly materials, such as starch-based compositions, are of interest in the manufacture of disposable containers and other articles, including cups, clam shell sandwich containers, plates, bowls, boxes and lids. Such compositions may include water, starch and other admixtures to improve processing and final properties. Starch-based plates and other containers manufactured by Biopac Corporation of Sweden were demonstrated at the winter Olympics in Lillihammer, Norway. Such containers were manufactured according to a process described in U.S. Pat. No. 5,376,320 to Tiefenbacher et al. For purposes of disclosing moldable starch-based compositions, the foregoing patent is incorporated herein by reference. Whereas the starch containers developed by Biopac were beneficial from an environmental standpoint, they were expensive to manufacture, both from a materials and processing standpoint.
Improved starch-based compositions that are less expensive to make and to mold into containers have been developed by researchers at E. Khashoggi Industries, LLC and are described in U.S. Pat. Nos. 5,662,731, 5,679,145 and 5,868,824. For purposes of disclosing moldable starch-based compositions, the foregoing patent is incorporated herein by reference.
In general, the manufacture of starch-bound articles having a foamed cellular matrix involves the use of aqueous molding compositions that include water, ungelatinized and pregelatinized starches, fibers, and other desired admixtures. The aqueous starch-based mixtures are introduced into a heated two-part mold in order to (1) gelatinize the ungelatinized starch portion and (2) remove at least a portion of the water by evaporation in order to yield a form stable molded article. The vaporizing water also causes the moldable composition to expand and fill the mold cavity. Venting means such as vent holes allow for escape of water vapor from the mold cavity and associated expansion of the starch-based mixture.
In order to ensure proper formation and density of the molded article, it is necessary to meter and deliver an optimum quantity of the moldable composition into the mold, typically a two-part mold including male and female mold halves. As stated above, the aqueous compositions typically expand to fill the mold cavity. If too little of the composition is introduced into the mold, it may not properly expand and fully fill the mold cavity, thus yielding incompletely formed articles which must be culled and discarded. On the other hand, introducing too much starch-based composition into the mold may result in excessive pressure build-up within the mold cavity, which can also result in improperly formed or damaged articles. In addition, excess mold material expelled through the vent holes typically remains attached to the molded article as unwanted xe2x80x9cflashingxe2x80x9d that must be removed and which can cause scarring or breakage of the article at the point where the flashing is broken off. In view of this, one would expect the failure rate when molding starch bound articles to be greatly reduced the amount of composition that introduced into the mold each time can be more precisely metered and delivered.
In a typical process the starch-based composition is mixed together batch-wise and then pumped through hoses to the mold site where it is intermittently dropped or expelled into the molds. One system for introducing the starch-based material into the molds consists of an aperture and shutter arrangement. Initially, the shutter covers the aperture and prevents the pressurized starch-based composition from exiting the aperture. When it is desired to introduce a quantity of material into the mold, the shutter is opened for predetermined duration and then closed. In theory, the speed and duration of this opening and closing action can be calibrated in relation to the pressure and rheology of the molding composition in order to introduce a desired amount of the composition into the mold each time. In reality, there are simply too many changing variables for such a system to accurately and precisely deliver the desired quantity of material into the molds during each and every cycle.
The inability of the aperture/shutter valve system to precisely and accurately deliver the same quantity of material during each cycle is due to a number of changeable variables. One variable is the pressure of the starch-based composition being delivered to the mold system. Assuming that the shutter speed and open dwell time are calibrated according to the specific mold requirements and the assumed pressure of the starch-based composition, even slight variations in the actual pressure of the starch-based composition can cause deviations in the amount of material actually delivered. If the pressure is higher than what is assumed, too much of the starch-based composition will be delivered; conversely, if the pressure is lower than what is assumed, too little of the material will be delivered.
Another variable is the rheology of the starch-based composition, including both the viscosity and yield stress of the material. Once again, even slight variations of mixture rheology can affect the flow rate of the material through the aperture. Such discrepancies in flow rate are only exacerbated when multiple valves are used, since it is difficult to ensure that every value experiences the same pressure and associated flow rate. Ensuring an even flow rate throughout all the valves would require a very complicated delivery system.
One way to overcome these problems might be to have a computer monitored system that could continuously monitor the foregoing variables and adjust the individual valve shutter speeds in order to maintain close tolerances of the quantity of material being delivered. In practice such an arrangement would be difficult to implement, particularly because the correction mechanism would only be triggered when deviations were actually detected. Such variations will likely have already resulted in poorly formed containers before corrective measures can be made. Moreover, a highly complicated system of measuring transducers, information relays, computer algorithms, individual speed and timing adjustments to each valve, and other required equipment for such a correction system would be extremely complicated and would tend to further increase the already high initial capital costs of setting up a manufacturing line. Moreover, careful calibration of all the equipment would be necessary from time to time in order to ensure that the information feedback and correction system would actually work as intended.
In view of the foregoing, what is needed are improved methods and systems for metering and delivering a precisely measured quantity of a flowable composition into a mold.
It would be a further improvement in the art to provide methods and systems for metering and delivering a precisely measured quantity of a flowable composition which did not change in response to fluctuations in pressure and rheology of the mold material.
It would also be an improvement in the art to provide methods and systems for metering and delivering a precisely measured quantity of a flowable material without regard to the number of valves and without regard to whether they are arranged in series or in parallel in relation to the pressurizing means.
It would additionally be an improvement in the art to provide methods and systems for metering and delivering a precisely measured quantity of a flowable material which avoided, or at least reduced, the problems inherent in both under filling and over filling the mold.
More particularly, it would be a tremendous advancement in the art if such methods and systems prevented or reduced the incidence of inadequately formed or collapsed articles and well as the quantity flashing that is attached to the demolded articles.
Such methods and systems for metering and delivering a precisely measured quantity of a flowable material to a mold apparatus are disclosed and claimed herein.
The present invention relates to apparatus and systems for providing a precise quantity of a flowable mold material into a mold during the fabrication of molded articles. In particular, the present invention relates to systems that allow for the precise, repeatable delivery of a predetermined quantity of a flowable material, particularly an aqueous starch-based composition, into a mold during each molding cycle in the manufacture of starch-bound articles. The ability to precisely meter and deliver a desired quantity of the mold material results in a more efficient molding process, with fewer articles being damaged due to under- or over-filling the molds and less excess flashing being produced during the molding process.
In general, the inventive metering and delivery systems according to the invention include three basic sections: (1) mixing means for mixing together a desired molding composition that will be inserted into the mold apparatus; (2) accumulation means for receiving and temporarily storing one or multiple batches of the mold material; and (3) delivery means for metering and delivering a precise quantity of the mold material from the accumulation means into an appropriate mold apparatus. These subsystems may advantageously be integrated to yield a fully automated metering and delivery system.
The mixing means typically includes a mixing vessel and a mechanical stirring device capable of imparting a desired level of high or low shear mixing. The mixing means may be capable of producing a desired mold composition batch-wise or on a continuous basis. In the case of a mixer capable of continuous feeding and mixing of mold mixture components, the resulting mold material may be continuously fed or pumped to an intermediate storage unit, or it may be delivered directly to the mold apparatus. In the case of a mixer that produces mold material batch-wise, the mold material will typically be transferred in batches to a hopper, which serves as a reservoir from which mold material can be continuously fed or pumped as needed within the overall molding system.
The accumulation means comprises a reservoir or accumulator capable of continuously and reliably supplying mold material under sufficient pressure to ensure that a continuous supply of mold material is available under pressure to the delivery means. The accumulation means is preferably capable of temporarily storing enough mold material for a sufficiently large number of delivery cycles without having to be recharged so that it does not prematurely run out of material while the molds are being charged with mold material. A pump or other pressurizing system may preferably be used to transfer mold material from the mixing means to the accumulation means under pressure, such as by means of hoses, pipes or other appropriate conduits.
The delivery means is configured so as to be capable of measuring and delivering a precise quantity of mold material to the mold apparatus each cycle. The delivery means is in constant or periodic fluid communication with the accumulation means, which supplies mold material to the delivery means under pressure upon demand. The delivery is advantageously arranged relative to the mold apparatus so that it can charge a plurality of molds with mold material in a single pass or cycle.
In a preferred embodiment, the accumulation means comprises a cylindrical accumulator with a pneumatically driven piston disposed therewithin. The piston expels mold material from the cylindrical accumulator to the delivery means at a desired pressure. When being charged, the pressure on the piston is relaxed so that the pressure being exerted on the mold material from upstream pumps exceeds the countervailing pressure, if any, that is exerted by the piston. In this manner, the piston is caused to retract by the force of the mold material, which causes the cylindrically shaped accumulator to be charged with mold material. Thereafter, a valve seals off the accumulator from the upstream flow path to prevent backflow of the mold material upstream. Sufficient force is then applied to the piston in order to provide a pressurized supply of mold material to the delivery means upon demand.
In an alternative embodiment, the accumulation means may comprise a flexible accumulator bladder that is capable of expanding or contracting as needed to either receive mold material therein or expel mold material therefrom. The accumulator bladder may advantageously be pressurized, such by being housed within a pressurized chamber, in order to provide mold material to the deliver means under sufficient pressure. An advantage of a flexible accumulator bladder is that it may be recharged at the same time that it is delivering mold material to the delivery means. An advantage of the cylinder/piston accumulator is that it is better able to prevent the buildup of pockets of older mold material. This is particularly advantageous in the case where the mold material tends to lose desired properties over time.
It should be understood that the foregoing examples of accumulation means, namely the cylindrical accumulator and bladder are merely examples of accumulation means that may be employed. The accumulator may in fact have other cross-sectional shapes, such as oval, square, rectangular, and the like, together with a correspondingly-shaped piston or other mechanical expulsion means. In some cases, where the mold material has low viscosity and is able to readily flow by the force of gravity, it may be possible to deliver the mold material to the delivery means under sufficient pressure by means of gravity alone, such as by means of a vertical hopper. Of course, even a hopper can be pressurized, such as by means of pressurized air in the area above the mold material.
Regardless of the specific accumulation means that is employed, it will be preferable for the accumulation means to be configured so that older mold material from previous batches is replaced by newer mold material, which may be referred to as a xe2x80x9cfirst in/first outxe2x80x9d system. In other words, it is preferable that pockets of old mold material do not endlessly remain within the accumulator. Continuously cycling mold material from the accumulation means to the delivery means ensures that the mold material will reliably possess the desired rheological and compositional properties in the case where the mold material deteriorates over time.
The delivery means preferably comprises a cylinder and piston combination. The cylinder volume displaced by the piston stroke corresponds to the desired quantity of mold material that is delivered to the mold. The cylinder is first filled with mold material under pressure from the accumulation means. During the filling cycle, the pressure of the mold material from the accumulation means exceeds the countervailing pressure, if any, exerted by the piston. Once the cylinder has been filled with mold material, a pneumatic pinch valve or other appropriating valve means is advantageously employed to sever fluid communication between the cylinder and accumulation means.
During the delivery cycle, the piston exerts enough pressure onto the mold material to expel it out of a delivery orifice into an awaiting mold. The valve means disposed between the cylinder and accumulation means prevents backflow of mold material into the accumulation means. Once the piston has completed its predetermined stroke so as to deliver a predetermined volume of mold material to the mold, a pneumatic pinch valve or other appropriate valve means closes off the delivery orifice so as to prevent premature expulsion of mold material through the orifice during the subsequent filling cycle.
Instead of a cylinder/piston arrangement, the delivery means may alternatively comprise any chamber having a positive displacement device therein that is capable of retracting and advancing in order to measure and deliver a relatively precise quantity of mold material into the mold apparatus. For example, a chamber may be provided having a non cylindrical cross section and mechanical displacement device having a corresponding cross section.
It is therefore an object of the present invention to provide improved methods and systems for metering and delivering a precisely measured quantity of a flowable composition into a mold.
It is a further an object and feature of the present invention to provide methods and systems for metering and delivering a precisely measured quantity of a flowable composition which do not change in response to fluctuations in pressure and rheology of the mold material.
It is also an object of the invention to provide methods and systems for metering and delivering a precisely measured quantity of a flowable material without regard to the number of valves and without regard to whether they are arranged in series or in parallel in relation to the pressurizing means.
It is yet an object of the present invention to provide methods and systems for metering and delivering a precisely measured quantity of a flowable material in order to avoid problems inherent in both under filling and over filling the mold.
It is an additional object of the invention to provide metering and delivery systems that prevent or at least reduce the incidence of inadequately formed or collapsed articles, as well as the quantity flashing that is attached to the demolded articles.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.